Process for forming protective phosphate coatings on metallic surfaces



United States Patent PROCESS FOR FORMING PROTECTIVE PHOS- PHATE COATINGSON METALLIC SURFACES Hellmut Ley, Heinz Keller, Heinrich Anton KarlKopp, Werner Rausch, and Heinrich Fleischhauer, Frankfurt am Main,Germany, assignors to Parker Rust Proof Company, Detroit, Mich., acorporation of Michigan No Drawing. Application December 11, 1953,Serial No. 397,778

Claims priority, application Germany December 17, 1952 5 Claims. (Cl.1486.15)

The present invention relates to a process for forming protectivechemical coatings on the surfaces of metallic articles and moreparticularly relates to processes which are adapted to form phosphate ormixed phosphate-oxide coatings on ferrous surfaces.

In the customary processes which are utilized in forming protectivechemical coatings on metal surfaces such as zinc phosphate, manganesephosphates and iron phosphate coatings, it is conventional topreliminarily clean the metal surface to be coated and to contact thatcleaned metal surface with a hot, aqueous, acidic solution of theselected phosphate either by dipping or by spraying and maintaining thecontact between the solution and the surface for a sufficient time toform the desired coating. In the preliminary preparation of the metalsurface, it has been customary to remove heavy oxide coatings byimmersion in molten or fused alkali metal hydroxides and to follow witha series of acidic pickles formed from mineral acids or combinationsthereof. After pickling, the metal surfaces are rinsed and are ready forthe reception of the protective chemical coating. Even when thephosphate coating solutions contain oxidizing agents, immersionprocedures usually require 5 to minutes time to obtain the desiredcoating. The preparation, maintenance and handling of both alkaline andacidic pickling baths is well-known to be highly undesirable. Moreover,the bare corrosion resistance of such chemical coating is known to berelatively poor and if corrosion protection is important, it isdesirable to subsequently treat the metal surface by oiling or with achromic acid rinse.

The primary object of the present invention is to provide a process forforming protective chemical coatings on metallic surfaces in a reducedperiod of time, which coatings exhibit superior corrosion resistance incomparison to that obtained with coatings produced by heretoforeconventional procedures.

Another object of the present invention is to provide a process forforming protective chemical coatings on ferrous surfaces which eliminatethe necessity for utilizing conventional acidic pickling proceduresprior to the formation of the protective chemical coating.

Still another object of this invention is to provide a process whichenables step-wise deformation operations to be performed on ferrousarticles requiring annealing between successive drawing operations, withthe minimum of additional steps in the formation of protective chemicalcoatings after each annealing step. Other objects and advantageousfeatures of the invention will become apparent upon considering thepresent disclosure in its entirety.

In accordance with the present invention, it has been discovered thatthe above and related objects may be accomplished by heating the metalarticle to be coated to a temperature in the range of 100 C. to 400 C.prior to contacting the same with the coating solution, and contactingthe article to be coated in its heated con- ICC dition with the aqueousacidic phosphate solution. Whereas in the past it has been customary tooperate phosphate coating solutions at temperatures ranging from about100 F. to about 200 F. and to immerse ar ticles to be coated in suchsolutions for about 5 to 10 minutes, it has been found that superiorphosphate coatings may be formed in accordance with the method of thepresent invention after a contact period of about one minute. As anexample, a low carbon steel having a light oxide coating, induced fromthe atmosphere, was pickled to smoothness with an aqueous sulfuric acidsolution (15% by 'volume), rinsed and pre-heated in hot water to C. andthen introduced into an aqueous acidic zinc phosphate coating solutionat a temperature of 95 C. and allowed to remain therein for 5 minutes. Aportion of the same steel was heated to 200 C. and immersed in the samezinc phosphate coating solution in its heated condition and allowed toremain in the solution for 5 to 10 seconds. Both coated samples weretreated with a chromic acid rinse and subjected to a salt spray mist of3% sodium chloride for 20 hours and observed for rusting. The firstsample was found to have 60% of its surface covered with a mediumthickness of rust. The second sample was found to have only 40% of itssurface covered with a very light coating of rust.

The heating of the metallic article to be coated may be done in a mildoxidizing atmosphere if desired and the resultant coating which formsafter contacting the oxide coated surface with the phosphate coatingsolution will evidence the improved corrosion resistance which ischaracteristic of this invention so long as the oxide coating which isoriginally induced is relatively thin, tightly adherent, and not theflaky oxide scale which is induced by oxidation for extended periods oftime at elevated temperatures. Such oxide coatings may be formedintentionally in atmospheres containing controlled proportions of oxygenor in air, when both the time and temperature are regulated, or mayinherently be present from a previous treatment such as a mild annealingor warm-rolling or coiling procedure.

Inasmuch as the pre-heating of the surface to be coated and the contactthereof with the coating solution while in its heated condition enablesthe use of any thin, tightly adherent oxide coating which is on thesurface and eliminates any necessity for the conventional acidicpickling procedures, it will be appreciated that the greatest advantageis gained when this process is employed. It is possible, however, toobtain the benefits which accompany the introduction of the article tobe coated into the coating solution at an elevated temperature even whenthe article is coated with a heavy flaky oxide coating. In this case,the article should be heated to a temperature above 400 C. in air or acontrolled oxidizing atmosphere, and then subjected to a reducingatmosphere to eliminate the scale and leave the metal surface in ametallicly clean condition, and if the temperature employed for thereducing atmosphere treatment is higher than 400 C., the article shouldbe cooled below 400 C. in an inert atmosphere so that the metalliclyclean surface Will be maintained clean. The article can then becontacted with the phosphate coating solution while at a temperature ofC. to 400 C. to form the superior corrosion-resistant coatings of thisinvention.

The particular phosphate coating solution which is selected for use inthe process of this invention is not particularly important. Any aqueousacidic phosphate solution which is known to form a protective phosphatecoating on metallic surfaces may be used, including zinc phosphate,manganese phosphate, iron phosphate and alkali metal phosphates. One ofthe most important end uses for the coatings of this invention is forthe protection of the metal surface during metal deformation operations,

such as tube or wire drawing, and the selection of the best phosphatecoating solution will depend upon the requirements of the particularapplication. All the above types of phosphate solutions, with or withoutthe conventionally employed oxidizing agents, are suitable for this enduse.

In the drawing of metallic articles such as tubes or wires, it isconventional to employ a protecting phosphate coating in conjunctionwith a lubricant, which lubricant usually contains some fatty material.The present invention enables the utilization of such fatty material informing an oxide coating by heating, in a controlled atmosphere wherenecessary, and eliminates the necessity for solvent removal of the fattymaterial and conventional pickling procedures prior to the formation ofa new protective phosphate coating. All that is necessary is to oxidizethe fatty material to a thin, tightly adherent oxide coating andintroduce the oxide-coated article at a temperature of between 100 C.and 400 C. in the phosphate solution.

. The contact between the metal to be coated and the coating solutionmay be made by dipping the article in the solution or by spraying thesolution on the article. In

either case, the solution may be cold, at room temperature orpreliminarily warmed. It is preferred that the article have atemperature between 200 C. and 300 C. when it is contacted with thephosphate coating solution since these conditions produce the mostcorrosion resistant coatings in the shortest periods of time.

The following examples illustrate the invention in greater detail.

Example I Low carbon steel wire was annealed in air at temperatures of200 C. and 400 C. and while at these temperatures was drawn through anaqueous zinc phosphate solution containing 22 grams/liter P205, 35grams/liter NO: andl9.5 grams/liter zinc. For comparative purposes,other portions of the same wire were heated in air to 200 C. and 400 C.and allowed to cool. The same phosphate solution was heated to 95 C. andthe cooled sections immersed therein for 5 minutes and withdrawn. Thewires were then coated with the same rust resisting oil coating. All ofthe sections of wire were subjected to identical salt spray tests in 3%sodium chloride mist for hours and checked for corrosion. The wirestreated in accordance with the method of this invention showed tracequantities of rust on about 40% of their surface. The wire sectionstreated by immersion for 5 minutes showed light rust over the entiresurface and much heavier rust over about 40% of the surface.

Example II Low carbon sheet steel having some fatty material on itssurface was cleaned with trichloroethylene and dried. An aqueous zincphosphate solution was prepared having 11.5 grams/liter Zinc, 14grams/liter nitrate and 13 grams/liter P205 and the temperature of thesolution was raised to 100 C. and the cleaned metal immersed in theheated solution for 5 minutes and withdrawn, the phosphate coatedsurface being then treated with oil. Another. portion of the same sheetmetal was first wiped with trichloroethylene, then heated to above 450C. in a controlled atmosphere furnace which contained a mild oxidizingatmosphere, and the oxidized surface was then treated in a reducingatmosphere of hydrogen at 450 C. until the surface became clean. Themetal was then transferred to an inert atmosphere cooling chamber wherethe temperature was decreased to approximately 400 C. and the articlewas then immersed in the same aqueous phosphate solution as used beforefor one minute and withdrawn. The coated metal was then oiled in amanner similar to the oiling procedure used in connection with thearticle resulting from the 5 minute immersion. Both articles were thensubjected to a salt spray test of 92 hours duration in a chamberemploying a 3% sodium chloride fog spray for 15 minutes out of eachhour. The inspection of the article resulting from the 5 minuteimmersion showed that the entire surface of the panel was covered withrust and that about 60% of the surface was covered with an extra heavylayer of rust whereas the article resulting from the method of thisinvention showed very light rust over the entire surface and only about20% of the area having a medium layer of rust thereon.

Example III An aqueous zinc phosphate solution was prepared containing10 grams/liter P205, 27 grams/liter zinc and 6 grams/liter chlorate. Lowcarbon steel preliminarily degreased was immersed in this aqueous zincphosphate solution at 100 C. for 5 minutes and withdrawn. Anotherportion of the same sheet steel was heated to 400 C. m a mild oxidizingatmosphere and at this temperature 1mmersed in the same solution for oneminute and withdrawn. Without applying oil or otherwise treating thecoating for the enhancement of the corrosion resistance, both portionsof the coated steel were subjected to a 3% sodium chloride fog spray fora 3 hour period. At the end of this time, the immersion process portionof the sheet steel was found to be completely covered with a mediumthickness layer of rust. The portion of the steel treated in accordancewith the method of this invention was found to have 20% of its surfacecompletely free of rust and the other covered with only a very lightlayer of rust.

What is claimed is:

1. A method for forming a chemical protective coating on metallicsurfaces which comprises the steps of heating the metallic surface in anoxidizing atmosphere to a temperature above 400 C. to form an oxidecoating thereon, heating the oxide coated surface in a reducingatmosphere to remove the said oxide coating and form a metallicly cleansurface, cooling said surface to a temperature in the range of C. to 400C. in an inert atmosphere, and contacting the said article in heatedcondition with a phosphate coating solution for a time suiiicient toform a phosphate coating on said surface.

2. A method for forming a chemical protective coating on ferroussurfaces which comprises the steps of heating the ferrous surface in anoxidizing atmosphere to a temperature above 400 C. to form an oxidecoating thereon, heating the oxide coated surface in a reducingatmosphere to remove the said oxide coating and form a metallicly cleansurface, cooling said surface to a temperature in the range of 200 C. to300 C. in an inert atmosphere, and contacting the said article in heatedcondition with a phosphate coating solution for a time suflicient toform a phosphate coating on said surface.

3. A method for forming a protective chemical coating on metallicsurfaces which have previously been mechanically deformed whichcomprises the steps of heating the said surface to a temperature above400 C. slowly cooling said surface in an oxidizing atmosphere to atemperature in the range of 100 C. to 400 C., and contacting the saidsurface while in the heated condition with an aqueous acidic phosphatesolution for a time sufficient to form on said surface a coating whichis at least partially a phosphate coating, and thereafter mechanicallydeforming said surface.

4. A method for forming a protective chemical coating on ferroussurfaces which have previously been mechanically deformed whichcomprises the steps of heating the said surface to a temperature above400 C. slowly cooling said surface in an oxidizing atmosphere to atemperature in the range of 200 C. to 300 C., and contacting the saidsurface while in the heated condition with an aqueous acidic phosphatesolution for a time sufiicient to form on said surface a coating whichis at least partially a phosphate coating, and thereafter mechanicallydeforming said surface.

A method for forming a protective chemical coating on ferrous surfaceswhich comprises the steps of forming a phosphate coating on saidsurface, contacting said phosphate coated surface with a lubricantcontaining fatty material, mechanically deforming said surface, heatingsaid mechanically deformed surface in an oxidizing atmosphere, andcontacting the said re-heated surface with an aqueous acidic phosphatesolution for a time suflicient to form on said surface a coating whichis at least par tially a phosphate coating.

6 References Cited in the file of this patent UNITED STATES PATENTS1,761,186 Baker et a1. June 3, 1930 2,543,710 Schmidt et a1 Feb. 27,1951 2,588,234 Henricks Mar. 4, 1952 OTHER REFERENCES Metals Handbook,1948 edition, page 6.

1. A METHOD FOR FORMING A CHEMICAL PROTECTIVE COATING ON METALLICSURFACES WHICH COMPRISES THE STEPS OF HEATING THE METALLIC SURFACE IN ANOXIDIZING ATMOSPHERE TO A TEMPERATURE ABOUT 400*C. TO FORM AN OXIDECOATING THEREON, HEATING THE OXIDE COATED SURFACE IN A REDUCINGATMOSPHERE TO REMOVE THE SAID OXIDED COATING AND FORM A METALLICLY CLEANSURFACE COOLING SAID SURFACE TO A TEMPERATURE IN THE RANGE OF 100*C. TO400*C. IN AN INERT ATMOSPHERE, AND CONTACTING THE SAID ARTICLE IN HEATEDCONDITON WITH A PHOSPHATE COATING SOLUTION FOR A TIME SUFFICIENT TO FORMA PHOSPHATE COATING ON SAID SURFACE.